Interface Processor

Annotation processing happens in a sequence of rounds. On each
round, a processor may be asked to process a
subset of the annotations found on the source and class files
produced by a prior round. The inputs to the first round of
processing are the initial inputs to a run of the tool; these
initial inputs can be regarded as the output of a virtual zeroth
round of processing. If a processor was asked to process on a
given round, it will be asked to process on subsequent rounds,
including the last round, even if there are no annotations for it
to process. The tool infrastructure may also ask a processor to
process files generated implicitly by the tool's operation.

Each implementation of a Processor must provide a
public no-argument constructor to be used by tools to instantiate
the processor. The tool infrastructure will interact with classes
implementing this interface as follows:

If an existing Processor object is not being used, to
create an instance of a processor the tool calls the no-arg
constructor of the processor class.

Next, the tool calls the init method with
an appropriate ProcessingEnvironment.

As appropriate, the tool calls the process
method on the Processor object; a new Processor
object is not created for each round.

If a processor object is created and used without the above
protocol being followed, then the processor's behavior is not
defined by this interface specification.

The tool uses a discovery process to find annotation
processors and decide whether or not they should be run. By
configuring the tool, the set of potential processors can be
controlled. For example, for a JavaCompiler the list of candidate processors to run can be
set directly or controlled by a search path
used for a service-style
lookup. Other tool implementations may have different
configuration mechanisms, such as command line options; for
details, refer to the particular tool's documentation. Which
processors the tool asks to run is a function
of the types of the annotations present
on the root elements, what annotation types a processor
supports, and whether or not a processor claims the annotation types it processes. A processor will be asked to
process a subset of the annotation types it supports, possibly an
empty set.
For a given round, the tool computes the set of annotation types
that are present on the elements enclosed within the root elements.
If there is at least one annotation type present, then as
processors claim annotation types, they are removed from the set of
unmatched annotation types. When the set is empty or no more
processors are available, the round has run to completion. If
there are no annotation types present, annotation processing still
occurs but only universal processors which support
processing all annotation types, "*", can claim the (empty)
set of annotation types.

An annotation type is considered present if there is at least
one annotation of that type present on an element enclosed within
the root elements of a round. For this purpose, a type parameter is
considered to be enclosed by its generic
element. Annotations on type uses, as opposed to
annotations on elements, are ignored when computing whether or not
an annotation type is present.

An annotation is present if it meets the definition of being
present given in AnnotatedConstruct. In brief, an
annotation is considered present for the purposes of discovery if
it is directly present or present via inheritance. An annotation is
not considered present by virtue of being wrapped by a
container annotation. Operationally, this is equivalent to an
annotation being present on an element if and only if it would be
included in the results of Elements.getAllAnnotationMirrors(Element) called on that element. Since
annotations inside container annotations are not considered
present, to properly process repeatable annotation types,
processors are advised to include both the repeatable annotation
type and its containing annotation type in the set of supported annotation types of a
processor.

Note that if a processor supports "*" and returns true, all annotations are claimed. Therefore, a universal
processor being used to, for example, implement additional validity
checks should return false so as to not prevent other such
checkers from being able to run.

If a processor throws an uncaught exception, the tool may cease
other active annotation processors. If a processor raises an
error, the current round will run to completion and the subsequent
round will indicate an error was raised. Since annotation processors are run in a
cooperative environment, a processor should throw an uncaught
exception only in situations where no error recovery or reporting
is feasible.

The tool environment is not required to support annotation
processors that access environmental resources, either per round or cross-round, in a multi-threaded fashion.

If the methods that return configuration information about the
annotation processor return null, return other invalid
input, or throw an exception, the tool infrastructure must treat
this as an error condition.

To be robust when running in different tool implementations, an
annotation processor should have the following properties:

The result of processing a given input is not a function of the presence or absence
of other inputs (orthogonality).

Processing the same input produces the same output (consistency).

Processing input A followed by processing input B
is equivalent to processing B then A
(commutativity)

Processing an input does not rely on the presence of the output
of other annotation processors (independence)

The Filer interface discusses restrictions on how
processors can operate on files.

Note that implementors of this interface may find it convenient
to extend AbstractProcessor rather than implementing this
interface directly.

Method Detail

getSupportedOptions

Returns the options recognized by this processor. An
implementation of the processing tool must provide a way to
pass processor-specific options distinctly from options passed
to the tool itself, see getOptions.

Each string returned in the set must be a period separated
sequence of identifiers:

SupportedOptionString:

Identifiers

Identifiers:

Identifier

Identifier.Identifiers

Identifier:

Syntactic identifier, including keywords and literals

A tool might use this information to determine if any
options provided by a user are unrecognized by any processor,
in which case it may wish to report a warning.

Returns:

the options recognized by this processor or an
empty collection if none

getSupportedAnnotationTypes

Returns the names of the annotation types supported by this
processor. An element of the result may be the canonical
(fully qualified) name of a supported annotation type.
Alternately it may be of the form "name.*"
representing the set of all annotation types with canonical
names beginning with "name.". Finally, "*" by itself represents the set of all annotation types,
including the empty set. Note that a processor should not
claim "*" unless it is actually processing all files;
claiming unnecessary annotations may cause a performance
slowdown in some environments.

Each string returned in the set must be accepted by the
following grammar:

process

Processes a set of annotation types on type elements
originating from the prior round and returns whether or not
these annotation types are claimed by this processor. If true is returned, the annotation types are claimed and subsequent
processors will not be asked to process them; if false
is returned, the annotation types are unclaimed and subsequent
processors may be asked to process them. A processor may
always return the same boolean value or may vary the result
based on chosen criteria.

The input set will be empty if the processor supports "*" and the root elements have no annotations. A Processor must gracefully handle an empty set of annotations.

Parameters:

annotations - the annotation types requested to be processed

roundEnv - environment for information about the current and prior round

Returns:

whether or not the set of annotation types are claimed by this processor

getCompletions

Returns to the tool infrastructure an iterable of suggested
completions to an annotation. Since completions are being asked
for, the information provided about the annotation may be
incomplete, as if for a source code fragment. A processor may
return an empty iterable. Annotation processors should focus
their efforts on providing completions for annotation members
with additional validity constraints known to the processor, for
example an int member whose value should lie between 1
and 10 or a string member that should be recognized by a known
grammar, such as a regular expression or a URL.

Since incomplete programs are being modeled, some of the
parameters may only have partial information or may be null. At least one of element and userText
must be non-null. If element is non-null, annotation and member may be null. Processors may not throw a NullPointerException
if some parameters are null; if a processor has no
completions to offer based on the provided information, an
empty iterable can be returned. The processor may also return
a single completion with an empty value string and a message
describing why there are no completions.

Completions are informative and may reflect additional
validity checks performed by annotation processors. For
example, consider the simple annotation:

@MersennePrime {
int value();
}

(A Mersenne prime is prime number of the form
2n - 1.) Given an AnnotationMirror
for this annotation type, a list of all such primes in the
int range could be returned without examining any other
arguments to getCompletions: